Thermal spray deposition is widely used to form coatings of a material on a substrate such as ceramic thermal barrier coatings (TBC) for gas turbines for power generation and aircraft engines. In the process, particles of powerdered feedstock material are entrained in a jet of high temperature gas or plasma directed at the substrate. The current coating practice is to spray feedstock material particles on a target part for a predetermined length of time. Thickness of the coating on the target part is then measured to determine if sufficient material has been applied. Parts which do not meet the quality specification (including thickness) must then undergo rework. Variations in the coating thickness from part to party have typically been due to torch aging and other variations of the deposition process.
Thermal spray processes have used a torch that produces a high-speed, high temperature jet of gas or plasma. When feedstock particles have been entrained in the jet with an injector mechanism, they both quickly accelerate toward the target and heat by absorbing thermal energy from the jet gas or plasma.
The trajectory any given particle follows once entrained in the jet is a function of the size of that particular particle, the initial velocity of the particle upon entering the jet, and state of the jet at that instant in time. These factors, however, vary from particle-to-particle and rapidly fluctuate with time. For example, Zirconia particles are typically supplied as a powder with diameters ranging from 10 μm to 90 μm and exhibit trajectories which deviate from the centerline of the jet by about 10 mm. Particle temperatures within the plume typically range from 100° C. to 4000 C. However, since the melting temperature of Zirconia is approximately 2700 C, many particles will not melt and will therefore not adhere to the target part. Instead they will strike the target and bounce off.
Therefore, the situation exists where a large but unknown number of particles are present throughout a relatively large region of space and only some of them will end up contributing to the coating. Moreover, since the particles present in the plume at any one time may consist of a large range in diameter, not all particles will contribute the same volume to the coating.
The need for improved control comes from the variation in coating thickness and density observed by applicants in over 400 hours of production runs using a prior art manufacturing deposition process. In this process, flat substrates were sprayed with yttria stabilized zirconia at least once per shift in a manufacturing environment to provide a lower bound measurement of process variation. Variations of +/−15%, with an 8% standard deviation in coating thickness'were observed, as shown in FIG. 1. In the graph dotted vertical lines indicate torch rebuilds and heavy horizontal lines indicate the average value of normalized coating thickness measured between these rebuilds. These significant variations mean that the coating thickness specification window, the range of acceptable variation, would have to be quite large to avoid having to do significant re-work. If the acceptable range for a particular customer was narrower the rework would add costs from wasted spray booth and operator time, wasted powder, and the need to remove excess coating from some of the parts. Since actual production parts are curved, which increases variations, the coating thickness specification window is typically set at +/−20% of the average thickness.
A paper by Gevelber, M. A., C. Cui, B. Vattiat, Z. Fieldman, D. Wroblewski and S. Basu, “Real time control for plasma spray: sensor issues, torch nonlinearities, and control of coating thickness”, Proceedings of the 2005 International Thermal Spray Conference, 2005 pp. 667-672 (“the Gevelber paper”), showed that control over certain parameters has the potential to significantly reduce variations in coating thickness. But the results depend on selecting a plume property to control that correlates well with coating thickness.
The experimental data in the Gevelber paper showed that the amount of variation itself varied with the parameter being closed-loop controlled, as shown in FIG. 2. Using an individual particle sensor the authors measured particle temperature and velocity for particles in the plume. They found that control of ensemble averages of particle temperature (labeled Tp) or both temperature and velocity (labeled Tp+Vp) actually led to more variation compared to no control at all (labeled open-loop). In contrast, they found that control of the flux of molten particles in the plume and the plume position (labeled Dep and Yc) showed substantially less variation and reduced the standard deviation of the variation by a factor of 3-5.
Prior art sensors for plasma spray monitoring and control fall into two main categories. (1) Full-plume sensors and (2) Individual particle sensors. Full-plume sensors provide bulk average characteristics of some ensemble of particles in the plume or provide some distribution of one or more quantities across the plume. 2) Individual particle sensors provide temperature, velocity and diameter for each particle passing through a small control volume (typically less then 1 mm3).
Full-plume sensors have not been effective for control because their outputs do not correlate well with coating deposition characteristics, as shown in FIG. 3a, 3b from the Gevelber paper. FIG. 3a is a plot of the average temperature versus normalized cross sectional coating area for those particles observed by a conventional individual particle sensor that was translated to image the entire plume as well as averaged to reflect the output of a full plume sensor. Individual particle temperature measurements were then averaged to reflect the output that a full-plume sensor would have provided. FIG. 3b is a plot of the total intensity observed by a CCD array versus the average coating cross sectional area.
There are four reasons full-plume sensors do not provide effective control: (1) They do not capture the proper volume-weighting of individual particles that characterizes the coating buildup from individual particles since the average is based on the cross sectional area of only those particles that are able to be detected through the limitations of the dynamic range. (2) Since primarily those particles which are molten contribute to the coating, measurements of bulk average quantities are not indicative of the subset of particles which will determine the coating properties. Intensity or bulk temperature will tend to be skewed to larger, cooler particles, as evidenced by the fact that the temperature obtained from bulk sensors is often below the melting point for YSZ sprays. (3) The detectors used in the sensors do not have the dynamic range needed to sense all particles. The dynamic range is the ratio of the largest to smallest intensity signal the sensor can detect, which determines the intensity the sensor can detect at the same time from small hot particles to large cold particles. When the sensors are operated in a configuration that avoids saturation of the detectors by the largest and brightest particles, they may miss a significant number of smaller particles that many times comprise the critical subset of molten particles. In the conventional sensor schemes, the dynamic range requirements are not easily met since it is difficult to observe the light from small particles which contribute to the coating, along with the large intensity observed from cooler, larger particles. (4) Bulk average sensors are unable to detect relative particle position and therefore unable to spatially resolve particle characteristics which affect coating thickness uniformity, and as such they cannot be used to monitor and adjust plume position.
Individual particle sensors are too slow for control, since they need to be able to measure 100,000 to 1,000,000's of particles across the whole plasma plume (10-20 mm). But since conventional individual particle sensors have small measurement volume (less than 1 mm3), measure particles one-by-one, and require translation to measure particles across the whole plume, the measurement of a large number of particles would take a long time. Thus individual particle sensors have not been suitable for robust monitoring and control. In addition, the individual particle sensors are subject to the same limitations in dynamic range to determine the molten particle flux relative to the entire particle flux.
Thus, the individual particle sensor technique of the Gevelber paper has not been suitable for monitoring the state of the particles during production runs. Therefore, a better scheme is needed to monitor the state of the particles, and this scheme is provided in the present patent application.